The present invention relates to a small-size zoom lens system, and more particularly to a small-size zoom lens system which can be used in a camera or an electronic video camera, such as a compact lens shutter camera, that does not require a long back focus.
Most widely used zoom lenses for cameras which require no long back focus are a two-group zoom lens system which includes a first lens group having a positive focal length and a second lens group having a negative focal length in view of the fact that the entire length of the lens system is reduced at a wide angle setting.
There has also been proposed a three-group zoom lens system which comprises a first lens group having a positive focal length, a second lens group having a negative focal length, and a third lens group having a positive focal length.
The two-group zoom lens is made compact at the wide angle setting. However, since the first lens group generally has a diaphragm, F.sub.NO of the lens is increased at a telephoto setting. If the magnification ratio is increased, the zoom lens will become extremely dark.
The three-group zoom lens system which has been proposed is bright unlike the two-group lens system, but is not compact in size.
Japanese Laid-Open Patent Publication No. 63-25613 discloses a three-group zoom lens of improved compactness. The disclosed zoom lens, however, cannot have a large magnification ratio because its first group lens is fixed when the power of the zoom lens is varied. Actually, the maximum magnification ratio disclosed in this publication is 2.1.